This invention relates generally to electric energy conversion, and, more specifically, to a system and a method for resistive braking capability of small generator sets with low moments of inertia connected to an electric grid.
Distributed energy resource (DER) systems are small power generators, typically in a range from 3 kW to 10000 kW, that generate power from various sources and transfer the generated power to a grid connected to the power generators. The grid may be a power grid or an island grid. Further, the grid collects the power generated from multiple power generators and transmits the power to different locations or to one or more customer loads. Typically, the DER systems are an alternative to or an enhancement of traditional electric power systems. Small power generators may be powered by small gas turbines or may include fuel cells and/or wind powered generators, for example. The DER systems reduce the amount of energy lost in transmitting electricity because the electricity is generated very close to where it is used, perhaps even in the same building. DER systems also reduce the size and number of power lines that must be constructed. However, due to increased use of distributed power generation, many grid codes are requiring small generator sets to provide enhanced capabilities such as fault voltage ride through.
When a fault in the electric power system occurs, voltage in the system could drop by a significant amount for a short time duration (typically less than 500 milliseconds) until the fault is cleared. Faults such as a phase conductor being connected to ground (a ground fault) or a short circuiting of phase conductors may occur during lightning and wind storms or due to a transmission line being connected to the ground by accident. In the past, under inadvertent fault and large power disturbance circumstances, it has been acceptable and desirable for small generator sets to trip off line whenever the voltage drop occurs. Operating in this way has no real detrimental effect on the supply of electricity when penetration level of small power generator sets is low. However, as penetration of small generator sets in the electric power system increases, it is desirable for these small generator sets to remain on line and ride through such a low voltage condition, to stay synchronized with the electric grid, and to be able to continue supplying power to the grid after the fault is cleared. This is similar to the requirements applied to large power generator sets.
Therefore, it is desirable to determine a method and a system that will address the foregoing issues.